Pyridine is an important intermediate in the manufacture of agricultural chemicals, e.g., herbicides and pesticides, and pharmaceuticals, and is also useful as a solvent in the polymer and textile industries. Important derivatives of pyridine include, for example, nicotinic acid and nicotinamide (vitamins essential for human health), chlorpheniramine (an antihistamine), cetylpyridinium (a germicide and antiseptic), isoniazid (an important antitubercular drug), and Paraquat.RTM. (a herbicide).
Pyridines having one methyl group attached to the ring structure are called methylpyridines or picolines, and include 2- or .alpha.-picoline, 3- or .beta.-picoline, and 4- or .gamma.-picoline.
Pyridine and picolines can be obtained as by-products of the coal tar industry or coke manufacture. However, pyridine is found in only small amounts in coal tar, and a preferred method of obtaining pyridine is by chemical synthesis. Chemical synthesis typically relies on a catalytic gaseous reaction (condensation) between ammonia (or amines) and carbonyl compounds such as aldehydes or ketones. However, these chemical synthesis methods have historically suffered from disadvantages of low yields and poor selectivity, and short operation cycle and catalyst lifetime.
The term "base synthesis" is known and used in the field of pyridine chemistry to identify synthetic processes by which bases of pyridine and its alkylated derivatives are prepared by reacting aldehydes and/or ketones with ammonia in the gas phase using a heterogeneous catalyst. For example, the reaction of acetaldehyde with ammonia in the presence of heterogeneous catalysts at 350 to 550.degree. C. yields 2- and 4-methylpyridines (.alpha.- and .gamma.-picolines). As another example, acetaldehyde and formaldehyde, can be reacted with ammonia to yield pyridine and 3-methylpyridine. Such pyridine synthesis methods are described, for example, in U.S. Pat. No. 4,675,410 to Feitler and U.S. Pat. No. 4,220,783 to Chang et al.
Reaction of acetaldehyde or certain other low molecular weight aldehydes and ammonia either in the absence or presence of methanol and/or formaldehyde to yield pyridine and alkyl derivatives thereof has been carried out in the presence of amorphous silica-alumina composites containing various promoters. See, for example, U.S. Pat. Nos. 2,807,618 and 3,946,020. The yields of desired products using the latter catalysts have been poor. Alkylpyridines have also been synthesized, as reported in Advances in Catalysis, 18:344 (1968), by passing gaseous acetaldehyde and ammonia over the crystalline aluminosilicates NaX and H-mordenite. While initial conversion utilizing these materials as catalysts was high, catalyst deactivation by coking was rapid, providing a commercially unattractive system, characterized by poor catalytic stability.
Synthetic crystalline zeolites having an intermediate pore size as measured by the Constraint Index of the zeolite being between 1 and 12, e.g., ZSM-5, have been found to provide commercially useful yields and product selectivities. U.S. Pat. No. 4,220,783 was pioneer in this discovery, teaching synthesis of pyridine and alkylpyridines by reacting ammonia and a carbonyl reactant which is an aldehyde containing 2 to 4 carbon atoms, a ketone containing 3 to 5 carbon atoms or mixtures of said aldehydes and/or ketones under effective conditions in the presence of a catalyst comprising a crystalline aluminosilicate zeolite having been ion exchanged with cadmium and having a silica to alumina ratio of at least 12, and a Constraint Index within the range of 1 to 12.
Use of a ZSM-5 catalyst component in a fluidized or otherwise movable bed reactor is taught in U.S. Pat. No. 4,675,410. U.S. Pat. No. 4,886,179 teaches synthesis of pyridine by reaction of ammonia and a carbonyl compound, preferably with added hydrogen, over catalyst comprising a crystalline aluminosilicate zeolite which has been ion exchanged with a Group VIII metal of the Periodic Table. The crystalline aluminosilicate zeolite has a silica to alumina mole ratio of at least 15, preferably 30 to 200, a Constraint Index of from 4 to 12, e.g., ZSM-5, and the process provides a high and selective yield of pyridine.
U.S. Pat. No. 5,013,843 teaches addition of a third aldehyde or ketone to a binary mixture of aldehydes and/or ketones used in preparing mixtures of pyridine and alkyl-substituted pyridines in large scale continuous processes. In a preferred system, propionaldehyde is added to a binary mixture of acetaldehyde and formaldehyde to produce beta-pyridine and pyridine. The catalyst for this process is a crystalline aluminosilicate zeolite in the acidic form having a Constraint Index of from 1 to 12, e.g., ZSM-5.
The use of a ZSM-5 catalyst modified with metals such as thallium, lead, or cobalt has been shown to increase the selectivity of pyridine by Shimizu, et al. in Microporous and Mesoporous Materials, Vol. 21, pp 447-451 (1998).
However, despite recent advances, existing processes for producing pyridine and picolines suffer from the disadvantage that although commercially acceptable, catalyst losses due to poor mechanical strength of the catalytic medium result in high operating costs, and an attrition resistant catalyst for manufacturing pyridine, picoline or other alkylated pyridines would be desirable.
U.S. Pat. No. 5,110,776 discloses the use of small amounts of phosphate treated zeolite catalysts, for example ZSM-5, as additives in fluid catalytic cracking (FCC) processes to improve octane yield by producing olefins. The phosphate treated catalyst, in addition to having improved catalytic properties, also has improved attrition resistance. The use of phosphorus modified ZSM-5 fluid bed catalysts as additive catalysts to improve the olefin yield in FCC is also described in U.S. Pat. No. 5,389,232 and in U.S. Pat. No. 5,472,594.
U.S. Pat. No. 4,380,685 relates to a process for para-selective aromatics alkylation, including the methylation of toluene, over a zeolite, such as ZSM-5, which has a constraint index of 1-12 and which has been combined with phosphorus and a metal selected from iron and cobalt. U.S. Pat. No. 4,554,394 discloses the use of phosphorus-treated zeolite catalysts for enhancing para-selectivity in aromatics conversion processes.
Combining phosphorus with a molecular sieve, as well as steaming, have been disclosed as useful to effect controlled reduction in diffusivity and micropore volume and enhance the selective production of para-xylene by toluene methylation in International Publication Number WO 98/14415.
The use of a phosphate modified amorphous catalytic material to improve selectivity to 2-picoline and overall yield is disclosed in U.S. Pat. No. 3,932,431 to Minato et al.
In light of the aforementioned catalytic effect of phosphorus, it is quite remarkable that the inventors have discovered phosphorus treated molecular sieve materials used in processes for producing pyridine andlor picoline or other alkylated pyridines have minimal or no effect on the catalytic properties of the reaction, yet still exhibit improved mechanical properties.